Method of hardening steel



July7, 1942- w. H. MAYO ETAL 2,289,138

METHOD OF HARDEN`ING STEEL Filed Aug. 3, 1939 Patented July 7, 1942 s PAT-ENT OFFICE METHOD F HABDENING STEEL Warren H. Mayo,

Elmendorf, Worcester, United States Steel Corporation, a corporation of New Jersey Munhall, Pa., and Harold J.

Mass., asslgnors to Application August 3, 1939, Serial No; 288,246

(Cl. 14S-2.1)

6 Claims.

This invention relates to 'metallurgy and more particularly to a method of hardening -steel' of the so-called transforming or transformable type to obtain steel articles having an" inner hardened core surrounded exteriorly by a relatively soft and machinable case. o One of the objects of the invention is to provide hardened bars, rods and other articles comprised of steel with a machinable surface area. Still another object is to improve the manufacture of nuts, bolts, bars, rods and the like steel articles. Still another object is to provide a method of heat treating steelto obtain a hard core of relatively high strength in combination with a relatively soft machinable surface. Other objects and advantages will be apparent as the invention is more fully hereinafter disclosed.

Heretofore in the art it has been customary to harden steel known in the art as transforming steels by heating the same or articles 'comprised of the steel to temperatures4 atv which the steel is rendered fully austenitic and thereafter cooling the steel at such a rate as to inhibit transformation from occurring until the martensite forming temperature has been obtained, and to thereafter reheat the steel to some intermediate temperature thereby to anneal or soften the steel to the desired hardness. In this method of hardening steel considerable difliculty is encountered in obtaininga hardening extending throughout the entire cross-sectional area of the article, and in most cases the inner core area of the article, instead of transforming into martensite as desired, transforms into the softer pearlite or troostite structures. Many special steel compositions have been devised to extend the hardened area inwardly from the surface to a greater depth, but in most instances every steel composition is limited as to cross-sectional area within which hardening in accordance with this well recognized method may be accomplished.

Another disadvantage of this method of hardening is that there is considerable warpage and damage done to the-exterior surface of steel articles hardened by the method and surface finishing of the hardened article cannot generally be accomplished except by grinding. The present invention aims vto overcome these disadvantages.

Briefly stated, the present invention comprises surfacing the steel with a thermal insulating coating which is stable towards molten metal or hot oil or molten salt mixtures commonly employed in usual rapid cooling operations but unstable in aqueous solutions, heating the steel to the usual temperature above the critical range at which the steel is fully austenitic, transferring the steel into a heat zone maintained at a temperature above the temperature of rapid martensite formation but below the upper critical temperature, preferably Within the range F., holding the steel in this heat zone for a sumcient tinie interval to permit the exterior of the steel for a-determined distance inwardly from the surface to cool slowly through the transformation temperature and to-undergo transformation into relatively soft pearlite while the remaining core portion remains at a temperature above the transformation temperature and in its austenitic condition, and then quenching the steel in water to convert the austenite in the core into martensite. Thereafter the steel may be reheated to an annealing temperature and the hard martensitic core may be tempered or "softened to the hardness desired without altering the pearlite in the exterior portion of the steel.

Thus treated the exterior of the steel being comprised of relatively soft pearlite may be machined or otherwise cut and threaded irrespective of the hardness of the interior portion and the process may be so regulated as to obtain any reasonable depth of this soft pearlite to permit the exterior to be shaped or machined to desired dimensional sizes.

Whereas, as above indicated, we propose to rst surface the steel with a thermal insulating coating, we have found that by heating the steel under oxidizing conditions to the temperature at which the vsteel is fully austentic and by prolonging theheating suiiicently we may produce on most steels, except those containing relatively large amounts of chromium and nickel, n an oxide coating which is particularly adapted as a thermal insulating coating for the purposes of the present invention. Alternatively we have found that various mixtures of refractory materials, such as calcium and magnesium oxides, mixed with `various proportions of silica sand, chromite and th'e like refractories applied to the steel surface as a sludge or slurry and air dried thereon prior to heating to temperatures above the critical temperature are equally as effective as heat'insulating blanket materials as the metal oxide coating, which is preferred only because of its simplicity of application.

As an example of the present invention the application of the same to a steel having the following composition will be described:

This steel is one common in the art and in y the accompanying drawing the hardening characteristics of the steel when treated in accordance with the present invention with and without the thermal insulating coating is shown.

300 F.500 60 The curve of Fig. 1 shows the cross-sectional hardness of the steel article treated where the thermal insulating coating is present and the curve of Fig. 2 shows the cross-sectional hardness of the steel article treated in substantially the same manner where the thermal insulating coating is not present.

The steel articles treated comprise round bars having a diameter approximating 1% inches and a length approximating 6 inches. As indicated in the drawing, the two articles were heated to a temperature of about l450 F. for ten minutes; one under oxidizing conditions and the other under non-oxidizing conditions. Both bars were then cooled rapidly by quenching the same in a lead-bismuth bath maintained at 300 F. and were held in the bath for ten seconds. Then both bars were quenched in a brine solution at atmospheric temperatures.

From the hardness curves Figs. 1 and 2 the effect of the presence of the thermal insulating coating of oxides on one bar may be clearly noted. The hardness of the bar having the thermal insulating coating is for an extended distance inwardly relatively soft as compared to the hardness of the same surface area of the bar, not provided with the thermal insulating coating; and the core area of the bar provided with the thermal insulating coating is relatively hard as compared to the core area of the bar not provided with the thermal insulating coat- 1118.

In a great number of tests on a large number of steel compositions all of the well recognized transforming or transformable type we have found that substantially the same results are obtainable by appropriate modifications of time and temperature of heating, quenching and holding, which as one skilled in the art will readily appreciate will vary widely with respect to composition, size cf the article, austenitic grain size and the like factors, and the determination of the speciflc temperatures or times applicable to any given steel composition of any given size or shape is believed to be well within the scope of one skilled in the art, in the light of the disclosure hereinabove made and the specific embodiment hereinabove given.

'Ihe present invention appears to be particularly adapted for the treating of steel articles, such as steel shafting, jail bars, machinery parts, rivets and the like which require machining, polishing or similar types of surface modification following the hardening treatment.

Having broadly and specifically described the present invention and given one specific em-l bodiment of the same, it is believed that it is apparent that many modifications therein may be made without essential departure therefrom and all such modifications and adaptations of the same are contemplated as may fall within the scope of the following claims:

What we claim is: l. 'I'he method of hardening an article comprised of a transformable steel which comprises enclosing the article in a thermal insulating blanket which is distintegratable in aqueous solutions but non-disintegratable in molten metal baths or fused salts, heating the article. to a temperature at which the steel is converted to its austenitic condition, transferring the article rapidly into a heat zone maintained at a temperature in the range of temperatures below the upper critical temperature but above the martensite forming temperature, holding the article at a temperature in this said range of temperatures for a time interval at least suincient to permit the surface of the article for an extended depth inwardly from the surface to cool slowly through the transformation temperature but insufficient to result in cooling through said temperature of the core area of the steel, and then rapidly cooling the steel to a temperature below the martensite forming temperature.

2. The method of claim 1, wherein the article subsequently is reheated to a temperature adapted to temper martensite.

3. The method of treating an article comprised of steel of the transformable type to produce a soft surface-hard core structure therein which comprises heating the article under oxidizing conditions to a temperature at which the steel is converted fully to austenite for a time interval adapted to obtain said conversion and to form a metallic oxide coating on the said article, transferring the article into a heat zone having a temperature within the range of temperatures above about 300 F. but below about 900 F., holding the said article in said heat zone for a time interval atleast suincient to permit the surface area of the article for an extended depth inwardly to undergo transformation into ferritic transformation products other than martensite but insuiiicient to permit said transformation of the core area of the article and then rapidly cooling the article to temperatures below about 300 F. to convert the remaining core area into martensite.

4. The method of claim 3, wherein the article is subsequently reheated to a temperature and for a time interval necessary to obtain the desired tempering or softening of said martensite.

5. The method of claim 3, wherein said article is comprised of any steel composition known in the art as a transformable steel and wherein the alloy constituents of the same do not inhibit the formation of said oxide coating.

6. The method of claim 3, wherein said article is first surfaced with a refractory thermal insulating coating.

WARREN H. MAYO. HAROLD J. ELMENDORF. 

